(a) Field of the Invention
The present invention relates to an organic electroluminescence (EL) display, and in particular, to an organic EL panel, a display provided therewith and a driving apparatus and method thereof, capable of sufficiently implementing the gray display only with a simple organic EL display cell.
(b) Description of the Related Art
Examples of currently used displays are most widely employed cathode ray tubes (CRTs) and liquid crystal displays (LCDs) which are increasingly applied to computers. However, since the CRTs and the LCDs have the disadvantages that the CRTs are too heavy and have large volume and the LCDs are not bright, have poor lateral visibility, and have less efficiency, they do not meet users' requirements.
Therefore, many researchers try to develop cheaper, more efficient, thinner and lighter displays, and one of attractive displays as a next generation display is organic light emitting devices (OLEDs).
The OLEDs are based on electroluminescence (EL: light emission upon application of electricity) of specific organic materials or polymers. Since the OLEDs do not require back light, they can be established to be slim and easily manufactured at lower cost compared with the LCDs. In addition, the OLEDs have wide viewing angle and are bright compared with the LCDs, they have been vigorously researched worldwide.
FIG. 1 is a circuit diagram illustrating an example of a conventional organic EL driving device.
Referring to FIG. 1, a typical organic EL driving device includes a switching transistor Qs, a storage capacitor Cst, a driving transistor QD and an organic EL element OLED.
The organic EL display is driven by active type driving with increased light-emitting duty but not by passive type driving emitting light only upon selection of one transverse scanning line, since the brightness of the organic EL display is low relative to the CRT. An active layer of the light-emitting cell emits light in proportion to an injected current density.
However, the driving transistor QD providing current for a light emitting element, i.e., the organic EL element OLED and the related EL element OLED connected to one terminal of the driving transistor QD have so wide distributions of voltage versus brightness characteristic that the organic EL display has a trouble in displaying grays.
In order to realizing uniform gray display but be little influenced by such distributions of voltage versus brightness characteristic, a kind of time-divisional gray display called a Display-Period-Separated (hereinafter, referred to as “DPS”) driving is used.
However, the DPS driving is disadvantageously limited to a display product with a few grays and low resolution, since the driving frequency may not be increased due to limitation of the operating speed of the switching transistor Qs, and it is hard to improve the brightness due to the relatively small light-emitting display time resulted from long data scanning time.
In order to solve these problems, a Simultaneous-Erasing-Scan (hereinafter, referred to as “SES”) driving is used, which improve the brightness by enlarging the display duty relative to the DPS driving.
FIG. 2 shows another example of another conventional organic EL device, and in particular, an example of an organic EL device suitable for application of the SES driving.
Referring to FIG. 2, an exemplary organic EL driving device includes a first switching transistor Qs1, a second switching transistor Qs2, a capacitor Cst, a driving transistor QD and an organic EL element OLED. In the figure, a first terminal of the driving transistor QD is connected to ground GND and a second terminal of the driving transistor QD is connected to one terminal of the organic EL element OLED with the other terminal applied with a driving voltage (−Vee) of negative polarity. Alternately, the first terminal of the driving transistor QD is applied with a driving voltage +Vee of positive polarity and the other terminal of the organic EL element OLED is connected to the ground GND.
However, the EL driving device shown in FIG. 2 additionally includes the second switching elements QS2 in respective organic EL cells, and further includes signal lines (ES line) for applying data erasing signals to the gate terminals of the second switching elements QS2 and row driving ICs generating the data erasing signals numbering in relation to a vertical resolution of the organic EL panel.
Therefore, there is a problem that this decreases productivity, thereby resulting in the increase of the product cost of the organic EL display.
Furthermore, there is a problem that the addition of the second switching transistor QS2 of a MOS type results in reduction of aperture ratio of the organic EL display cell to decrease the brightness and light-emitting efficiency.